This invention relates to degradable agricultural mulch films. More particularly, this invention relates to mulch films that are stable to weathering conditions for a predetermined period and then rapidly decompose.
Applying plastic film for agricultural mulching has increased rapidly the past few years. Vegetables, tomatoes, and strawberries are among the more common crops on which plastic mulch is used. Principal benefits of mulching are to provide weed control, warm the soil for early crop production, control soil moisture, and reduce nutrient leaching.
Polyethylene film is the most common plastic mulch; however, it must be removed from the field and burned or buried at the end of each fruiting season since it does not decompose in time to start the subsequent crop. In fact, the plastic persists for several years if not removed.
Because the removal and burying or burning of plastic mulch is both costly and has an adverse effect on the ecology, the need for a plastic mulch that will decompose at the end of a growing season has recently become apparent. The preferred mulch life of a film depends upon the individual crop. For tomatoes we estimate that the film should remain intact for 3 to 4 mo.; for lettuce, radishes, and other quick maturing crops with a one-time harvest, the film needs to last only a few weeks. In many areas where these crops are grown the climatic conditions are suitable for growing two or more crops per year on the same plot. Here it would be particularly advantageous to use plastic mulch that will rapidly deteriorate after a given time of use and not interfere with soil preparation and planting of the new crop.
Numerous attempts have been made to produce a suitable degradable mulch [Chem. Wk. 110: 44 (1970)] including polyethylene-coated paper [HortScience 7: 568 (1972)] and polybutene- 1 films (U.S. Pat. No. 3,590,528). None has been completely successful, apparently because they are too costly or they decompose too slowly for many applications.
We have developed a coated starch-based film that has strength and flexibility properties suitable for mulch applications but will degrade after a given time of weathering conditions--depending upon the amount of coating.
Although whole starch has been investigated for many years as a potential raw material for nonsupported films, it has never been successful because its films are brittle and are greatly affected by moisture [Cereal Chem. 40: 154 (1963)]. Large amounts of compatible plasticizers, such as glycerol or ethylene glycol, are effective softening agents for whole starch, but the films resulting are too soft and tacky at high humidity and have virtually no wet strength. These problems were overcome in this invention by preparing a starch-poly(vinyl alcohol) (st-PVA) film and coating the film with a water-resistant coating. Starch has been used with poly(vinyl alcohol), particularly in adhesives, textile sizes, water-insoluble films, and water-soluble films (for example: U.S. Pat. Nos. 3,652,541; 3,652,542; 3,472,804; 3,425,972; and 3,316,190).
In accordance with the invention, we have discovered a self-supporting, degradable agricultural mulch film comprising from 1 to 4 parts of poly(vinyl alcohol) (PVA) having a degree of hydrolysis of from 85 to 100%, 8 parts of starch, and from 1 to 5 parts of glycerol; said film being completely coated with from 5 to 20% of a water-resistant coating composition comprising a mixture of from 0.1 to 0.2 parts of a suitable polyol-toluene-diisocyanate prepolymer and 1 part of poly(vinylidene chloride-acrylonitrile) copolymer or 1 part of poly(vinyl chloride) resin containing a plasticizing amount of a suitable plasticizer.
The st-PVA film is coated to yield a composite film that resists weathering conditions associated with its use as an agricultural mulch for a controlled period and then rapidly deteriorates into small particles which mix harmlessly with the soil. The time at which decomposition occurs depends upon the thickness or amount of coating.
The reason for the controlled degradability of the starch-based film is not fully understood. However, it is believed to be due in part to the development of small fractures in the coating, due to exposure to weathering conditions, which in turn allow moisture to come in contact with and leach out the glycerol plasticizer. Without plasticizers starch films become brittle and rapidly deteriorate upon exposure to erosive forces. Furthermore, starch is known to rapidly biodegrade when exposed to soil microorganisms as would happen when soil moisture is permitted to pass through the coating.